In the electrophotography heat-pressure fixing method in which an image folding material having an image formed with toner is transferred between heating roller and pressure roller to fix the image, is widely employed because the apparatus therefor is simple and fixing property to a support such as a paper is satisfactory. In this method transmission of heat to the toner is caused by contact with the heating roller, and toner is fused by the heat. This method has disadvantage to tend to bring offset phenomenon, which fused toner adheres to the heat roller, as fused toner contact with the heating roller.
Various means for improving image disarrange caused by the offset have been proposed. In general the offset is explained as it occurs in case that adhesive force between the toner and heating parts such as heating roller is greater than the inner coagulation force. On the other hand, many methods of addition of agent giving releasing ability into a toner are proposed in view of the adhesion to heating device. Further, methods to prevent offset phenomenon by coating silicone oil etc. on a heating device is also proposed. These method demonstrate the effect by employed solely or in combination.
There is another problem of fixing property in heat-pressure fixing, which adhesion characteristics to a transfer material such as paper. There are methods employing with high fixing temperature or rather induce offset phenomenon. For these reasons, many improvement methods have been proposed of resins as for fixablity in view of viscosity when the toner melts.
It is a extremely important problem how to obtain wider fixable temperature range between the fixable minimum temperature and the temperature at which offset occurs. The problem has not been dissolved satisfactory, particularly, for small particle size toner and color tone.
There have been other subjects such as obtaining high quality image at initial stage, preventing of grade down of image quality as repeating employing, and preventing poor image in a process utilizing electrostatic image developing toner. For example, the problems includes gradation characteristics, reproduction ability of fine lines, change of image density, uneven image density, fogging. These are manly caused by unstable toner charging quantity, which is difficult in controlling. Stabilizing and controlling of charge quantity of toner are extremely difficult as the charging is caused triboelectrically.
Various kinds of improvement such as binder resin for toner, charge control agent, external additives, other additives and so on, have been proposed for above mentioned problems. However, further improvement in higher image quality and higher durability of developer are demanded in accordance with the progress of performance and reliability in each step of image forming process utilizing the toner.
In recent years, the electrophotography has been applied to various fields. For example, printer for output terminal of computer, color copying machine, color printer in addition to monochrome copying machine. As advancing of utility for the area, higher image quality is much more demanded. Variation of hue of secondary color by superposing color images becomes remarkable because of slight change of developing property (amount of residual toner) caused by very little change of charging characteristics etc., or change of transferring characteristics of half tone image in the image forming method in which toner images by color toner are superposed multiply. And consequently demand for stabilizing charging characteristics becomes extremely strict. Similarly stabilization of charging characteristics is demanded extremely strict in image forming method employing digital exposing which requires reproduction of fine lines.